1. Field of the Invention
The present invention relates to a method for plasma treatment in hollow bodies, in particular for plasma-chemical modification of surfaces which are located in the interior of such hollow bodies.
2. Description of the Related Art
Plasma treatment of surfaces plays a major role in many fields of technology. Thus, for example, thin coatings of widely different materials are deposited onto surfaces to be processed, by means of plasma-enhanced chemical vapour deposition (PECVD). The deposited coatings can in this case carry out different functions as an insulation coating, conductivity coating, diffusion barrier etc.
Plasma treatment in the interior of hollow bodies represents a special case group. For example, the plasma treatment of the internal surfaces of medical equipment, such as catheters, can be used to improve their biocompatibility. Such an application is disclosed, for example, in U.S. Pat. No. 5,244,654 or in U.S. Pat. No. 5,486,357.
A further method for plasma treatment is disclosed in U.S. Pat. No. 5,700,327. In the case of this method, an oxidizing process gas is passed into the interior of a container, while a low pressure is maintained in this container. A plasma is ignited in the interior by applying an electrical field to the container, and this plasma has a cleaning effect on the interior of the container.
U.S. Pat. No. 5,677,010 discloses a method in which a polymer coating is deposited in the interior of a fuel tank by means of a plasma-chemical process at reduced pressure. The fuel tank is located in a plasma process chamber, but is connected via a separate gas-tight line to a vacuum pump outside the process chamber. The pressure inside the tank is reduced, via this separate line, further than the pressure in the process chamber. At the same time, a process gas is passed through the tank at a defined speed, via a further line. Owing to the chosen pressure relationships, a plasma is ignited only inside the tank. However, considerable equipment complexity is required to carry out the method since the interior of the tank must be evacuated and supplied with process gas independently of the interior of the vacuumn chamber. This can be achieved only by means of additional gas-tight connections from outside the vacuum chamber to the interior of the hollow body.
A similar method is disclosed in U.S. Pat. No. 5,521,351, the pressure in the interior of the hollow body in this case being selected by metering the inlet and outlet of a process gas in such a manner that the conditions for a gas discharge (plasma) are created in the interior of the hollow body while, by permanent pumping out, the pressure in the process chamber is reduced in such a manner that no gas discharge is possible there. The plasma-chemical modification thus takes place only in the interior of the hollow body. Once again, the equipment complexity is considerable.
One specific application of a plasma-chemical process for sterilization of the internal surfaces of plastic packages is disclosed in U.S. Pat. No. 5,531,060. In this case, the packages are produced by a bubble moulding technique, in which the plastic material is pressed into a prepared mould under the influence of an appropriate gas pressure, this mould corresponding to the shape of the subsequent packaging. The document proposes that a gas which is suitable for the subsequent plasma treatment be used for the bubble moulding and that this gas be enclosed in the packaging by subsequent sealing. The packaging is then placed in a vacuum chamber, is opened at one or more points, and is evacuated together with the vacuum chamber. Finally, when a suitable pressure is reached, a plasma can be ignited in the packaging. In the case of this method, the production process is thus combined at this stage with the subsequent plasma treatment process. There is thus no longer any need to supply process gas in addition to the vacuum chamber. However, the necessity to open the packaging inside the vacuum chamber presents considerable problems.
The object of the present invention is to provide a method for plasma treatment in hollow bodies, which can be used easily and with little equipment complexity, without any modifications to existing plasma systems.